The Monoclinic Pyroxenes of Neiv York State. 175 



The optical constants of the Russel diopside were determined 

 to be, 



D:r 



37° 





Na 



a 



1.6626 



/5 



1.6718 



7 



1.6940 



y-a 



.0314 



2W 



77° 34' 



2V 



58°5 6' 



Parallel growths of amphibole and pyroxene are common at 

 this locality and the late Prof. G. H. Williams has called attention 

 to the fact that the lessei' extinction angle always lies on the 

 same side of the vertical axis. Prof. Williams has also described 

 and figured a remarkable growth of dark green hornblende from 

 Russel, around a crj-stal of pale green pyroxene. Both crj^s- 

 tals have the clinopinacoids parallel, as is also the parting which 

 is present in both. The specimen is about three inches long and 

 is in the collection of Mr. C. Bement of Philadelphia. 



Genesis of the New York Pyroxenes. 

 As previously stated, the pyroxenes occur under the following 

 conditions : 



1. As primary constituents of the igneous rocks. 



2. In contact zones between the igneous rocks and the lime- 

 stones. 



3. Disseminated through the limestones in regions which have 

 been subjected to dynamic metamorphism. 



4. Associated with the bodies of magnetite ore. 



The first case includes pyroxene occurring both in dikes and 

 intrusive masses, and the mode of formation is apparent, the 

 pyroxene representing one of the products of crystallization from 

 an igneous magna. It is among the earliest minerals to crystal- 

 lize out, and is preceded by magnetite, apatite, zircon and some- 

 times biotite. 



When thus occurring as a constituent of the igneous rock it 

 belongs to the variety augite, which is essentially a mineral of 

 igneous rocks. The experiments of Berthier, and Fouque and 

 Levy (Synthese des Mineralogie des Roches, Paris, 1882) have 

 shown that augite can be easily produced artificially from a fused 



